1. Field of the Invention
This invention relates to a gas analyzer provided with a multilayer interference filter on an optical path extending from a light source through a cell to a detector.
2. Description of the Prior Art
The multilayer interference filter used in a gas analyzer is usually a band-pass filter. It is important for such a bandpass filter to have a specific wavelength as a central wavelength because the specific central wavelength is the wavelength corresponding to the analyzer in which the filter is installed. Such band-pass filters are generally produced by vacuum depositing about 100 layers of a thin film which is about 0.05 .mu.m thick on a filter substrate. The accuracy of the film thickness is required to be 0.5% or less in order to obtain the required specific central wavelength.
From the standpoint of manufacturing technique for such a multilayer film interference filter, it is difficult to manufacture the filters with a specific central wavelength, and in practice the specific wavelengths can be determined only after manufacturing. Hence, the number of usable multilayer interference filters is very limited. Only a slight difference of the central wavelength from the desired wavelength requires discarding the filter. The manufacturing process thus has a low yield of filters. The multilayer interference filters are usually expensive, the low yield resulting in a high manufacture cost for each usable filters.